GB2088067A - Weighing or force measuring apparatus with load cell overload protection arrangement - Google Patents

Abstract

A load cell overload protection arrangement in a weighing or force measuring apparatus comprises a load limiting stop beam 26 connected at spaced-apart anchorage points 28', 28', to a main supporting beam 14 mounted upon a rigid base or frame 18 by mountings 16, 16, at opposite ends. The load cell 10 is connected to the mid-point of beam 14 and the applied load is transmitted thereto via a ring-like component 22 through with the stop beam 26 extends. Under load, supporting beam 14 deflects and the stop beam 26 and load cell 10 both move but to different extents so that a clearance gap is taken up and beam 26 engages an abutment face 30 when the rated maximum capacity is reached. Thereafter, excess load is transmitted direct to beam 14 through stop beam 26 and support rods 28 and the load on load cell 10 will increase only slowly, or remain constant, or diminish, dependent on the relative deflection characteristics of beams 14 and 26. <IMAGE>

Description

SPECIFICATION Weighing or force measuring apparatus with load cell overload protection arrangement This invention relates to weighing or force measuring apparatus, such as certain industrial weighing machines for example, incorporating load cells for providing an output signal indicative of the applied load.

Such load cells are generally designed to function in tension and/or in compression such that they undergo a dimensional variation dependent upon the load to which they are subjected, and they usually have a rated load capacity beyond which they are to be regarded as overloaded. If the overload is too great, permanent damage can be caused and in many cases with typical load cells in industrial weighing machines the maximum overload which can be applied before risk of permanent damage arises is about 50% above the rated load capacity.

For example, for a load cell with a rated capacity of 100 Ibs. the maximum overload it is likely to be able to withstand without permanent damage being caused may typically be about 1 50 Ibs. In practice, however, especially in an industrial environment, occasional misuse and application of an excessive overload is not uncommon. To try to avoid the permanent damage which would then result and the consequent expense and disruption to the user of having to carry out repairs, industrial weighing machines at least which incorporate load cells often include some kind of overload protection device.

Load cell overload protection devices in use in weighing machines commonly comprise a dead stop arrangement or shear pins. Dead stop arrangements may further be combined with either an initially unstressed spring or a preloaded spring in series with the load cell. However, each of such overload protection devices as hitherto known to us has certain practical disadvantages.

For instance, in the case of a simple dead stop arrangement which relies entirely on the movement or dimensional variation under load of the load cell, as the extent of the latter is small it is difficult to set the position of the dead stop with sufficient precision and there is oniy a very small working clearance or overload gap which is liable to become blocked or to entrap foreign bodies resulting in premature overload conditions and incorrect weight readings. Also, a dead stop mounted within the weighing machine or on its housing may move due to insufficient rigidity and distortion of the structure during use thereby causing the overload gap to vary.Disadvantages of the simple dead stop arrangement also still remain in the devices referred to which additionally include a spring and where the latter is initially unstressed its relative softness is liable significantly to slow down the response of the weighing machine while the use of a preloaded spring is often inconvenient in that such spring must usually be provided with accurately controlled and known characteristics.

All the above devices are, moreover, generally susceptible to shock loads, both in respect of the load cell itself at the instant when it is loaded and in respect of the weighing machine structure when the movement of the load cell suddenly bottoms against the dead stop. Furthermore, protective devices which rely on shear pins or the like are not entirely satisfactory since when a shear pin fails in overload conditions the weighing machine is then immobilised until a replacement pin has been fitted and the machine is tested and reset.

An object of the present invention is accordingly to provide an improved form of overload protection for load cells in weighing apparatus whereby various such disadvantages and difficulties encountered with the prior art overload protection devices as indicated above may be reduced or overcome.

According to the present invention, in a weighing or force measuring apparatus wherein the applied load is transmitted to a load cell and wherein the load cell is associated with a load limiting stop effective to provide overload protection by limiting the movement arising from the dimensional variation of the load cell under load, a common yieldable flexure member carries and supports, independently of one another and with different deflection characteristics, both the load cell and the stop in a live mounting arrangement whereby, under an applied load, the load cell and the stop both move in unison but to a different extent while the load is within the rated maximum capacity of the load cell so that there is relative movement between the load cell and the stop which causes an initial overload clearance gap to be taken up and the stop to be brought into operative abutting engagement when the load transmitted to the load cell reaches said rated maximum capacity, and in an overload condition, arising from a further increase in the load, the stop then transmits excess load direct to the flexure member thus protecting the load cell from excessive overloading.

Preferably, the flexure member is in the form of a beam to which the load cell and stop are connected at different points spaced apart along the length thereof to provide the different deflection characteristics. Such beam may be supported through mountings at opposite ends by a rigid base or frame of the apparatus and the load cell may be connected to the mid-point of the beam with the stop connected to anchorage points spaced either side of the mid-point of the beam. With this arrangement, in preferred embodiments the stop is also in the form of a beam in spaced parallel relationship with the main support beam and arranged to as to be engaged under overload conditions by an abutment on a member which transmits the applied load to the load cell.This abutment may conveniently be provided by an inner surface portion of a ring or stirrup form component of the load transmitting member and the overload stop beam is arranged to pass through the ring or stirrup component with, under zero applied load conditions, a predetermined clearance gap between it and the abutment surface portion of said ring or stirrup component.

This overload stop beam and its connections to the main support beam will have its own deflection characteristics and depending upon how these compare with those of the central portion of the main support beam to which the load cell is connected, under overload conditions either the load cell will receive no further loading, or the load cell will receive some additional load but substantially less than the excess load applied, or the load on the load cell will diminish.

By way of example, the manner in which the invention may be carried out will be more particularly described with reference to the embodiments illustrated in the accompanying drawings.

In said drawings, Figure 1 illustrates an overload protection arrangement for a tension type load cell in a weighing machine according to a first embodiment; and Figure 2 illustrates a similar arrangement modified for a compression type load cell in a weighing machine according to a second embodiment.

Referring first to the arrangement of Figure 1, this shows a load cell 10 connected at its upper end by a tension rod or cable 12 to the mid-point of a main support beam 14 which in turn is supported through flexible mountings 1 6 at opposite ends upon a rigid base or frame 18 of the weighing machine.

Load applied to the weighing machine is transmitted, as from a weight or transfer lever for example, through a connecting rod or cable 20, a ring-like stirrup component 22 and a further short connecting rod or cable 24 to the lower end of the load cell, as indicated, and passing through the stirrup component 22 is a second beam 26, termed an overload stop beam, that is also supported by the main support beam 14 to which it is connected by spaced-apart tie rods 28.

It will be noted that the anchorages 28' of the tie rods 28 on the main beam 14 are spaced equidistant either side of the mid-point and the tie rods are adjusted to be of equal length so that the overload stop beam 26 lies in parallel spaced relationship with the main beam. The central portion of beam 26 lies within the stirrup 22 with its top surfaces 28 spaced by a small gap from an inner abutment surface 30 of the stirrup provided by a downwardly presented terminal head or nut 32 of the connecting member component 24. This gap, forming an overload clearance gap, is set initially when adjusting the apparatus under zero load conditions to a particular predetermined value dependent on the known mechanical properties and deflection characteristics of the assembly, including those of the two beams and the characteristics and rating of the load cell.

In operation, when an applied load is transmitted through the load transmitting components the load cell 10 and main beam 14 are caused to deflect downwardly in the direction of the load. At the same time, the deflection of the main beam 14 causes the overload stop beam 26 to move down in unison but its movement is less that that of the load cell and stirrup component 22 which moves therewith so that the overload clearance gap decreases. If the load transmitted to the load cell then enters the overload range of the latter the overload clearance gap is closed and the abutment surface 30 engages the top surface 28 of the overload stop beam 26 so that any further increase in load will then be transmitted to this beam 26 and through the tie rods 28 direct to the main beam 14.This further deflects the main beam which bows downwardly about its midpoint so that the load cell is also moved downwards further relative to the spatial position of the stop beam anchorages 28' 28'. The stop beam 26 and its tie rods 28, however, will also defiect under the load transmitted therethrough and if the extent of this deflection is equal to that of the main beam 1 4 between the anchorages 28', 28', the load cell will see no further increase in load. If, on the other hand, the extent of this deflection is not equal to that of the main beam 14 between said anchorages, the load cell will see either positive or negative load increments in the overload region dependent on the relative deflection characteristics.

Thus, in the case of a relatively stiff overload stop beam which deflects less than the central portion of the main beam, in the overload region load will appear to be removed from the load cell thereby causing the readout to decrease the weight already recorded but giving full overload protection. With a less stiff overload stop beam which deflects more than the central portion of the main beam, in the overload region the load cell will still receive additional load but this will only be a fraction of the actual increase in load such that the overload protection is still adequate. Which of the three possible effects takes place is determined in the original design and choice of materials and deflection characteristics.

It will be appreciated that with the arrangement described overload protection can be obtained without significantly reducing weighing response time and in a manner which is related directed to the load cell. Also, the overload clearance gap can readily be set and adjusted. Moreover, since the load cell mounting and overload arrangement is yieldably supported, some degree of protection against shock loading is automatically obtained. If desired, further shock loading protection can be provided by fitting elastomer springs or other damping means in, or in association with, the assembly of the load transmitting components, for example within or attached to the stirrup component or connecting rod.

The arrangement illustrated in Figure 2 is very similar to that already described and functions in the same manner, except that the role of parts in compression and in tension are interchanged. The same references are used to denote corresponding or equivalent parts.

It will be understood that the arrangement can also readily be adapted, by combining the design features of the two embodiments illustrated, to give overload protection for universal load cells which operate under both tensile and compressive loads.

Claims (6)

1. A weighing or force measuring apparatus wherein the applied load is transmitted through load transmitting members to a load cell which is associated with a load limiting stop effective to provide overload protection, characterised in that the load cell and the load limiting stop are both carried and supported independently of one another, in a live mounting arrangement, by a yieldable supporting flexure member to which they are both connected so that, under an applied load, said flexure member deflects and the load cell and the stop both move in unison therewith, the arrangement being such that with an increasing load the load cell and the stop respectively move to a different extent thereby to cause an initial overload clearance gap to be taken up and the stop to be brought into operative abutting engagement with a said load transmitting member when the load transmitted to the load cell reaches the rated maximum capacity thereof, whereas in an overload condition, arising from a further increase in the load, the stop remains in operative abutting engagement with said load transmitting member and then transmits excess load direct to the supporting flexure member thus protecting the load cell from excessive overloading.

2. Apparatus as claimed in Claim 1 , wherein the supporting flexure member is in the form of a beam, constituting a main support beam, to which the load cell and stop respectively are connected at different points spaced apart along the length of the beam so as to have different deflection characteristics.

3. Apparatus as claimed in Claim 2, wherein the main support beam is supported at opposite ends by mountings upon a rigid base orframe of the apparatus and wherein the load cell is connected to the mid-point of the beam, whereas the stop is connected to anchorage points spaced either side of the mid-point of the beam.

4. Apparatus as claimed in Claim 3, wherein the stop is also in the form of a beam in spaced parallel relationship with the main support beam and arranged so as to be engaged under overload conditions by an abutment on said load transmitting member which transmits the applied load to the load cell.

5. Apparatus as claimed in Claim 4, wherein said abutment is provided by a portion of a ringlike or stirrup component of the load transmitting member and the overload stop beam is arranged to pass through such ring-like or stirrup component with, under zero applied load conditions, a predetermined clearance gap between it and the abutment portion of said ringlike or stirrup component.

6. A weighing or force measuring apparatus including a load cell and an associated load limiting stop substantially as herein described and illustrated with reference to Figure 1 or Figure 2 of the accompanying drawing.